BN Composites Research Articles

Borazine derived porous boron nitride–boron nitride composites fabricated by precursor infiltration and pyrolysis

Porous BN–BN composites were fabricated by unidirectional die pressing and precursor infiltration and pyrolysis (PIP) applying borazine as the precursor. The composition, mechanical, microstructural and dielectric properties of the composites were investigated. Pure hexagonal BN–BN composites with different properties can be obtained at temperatures between 1200°C and 1600°C. With increasing the pyrolysis temperature, the open porosity of the composites decreases whilst their bulk density and mechanical properties increase. When pyrolyzed at 1600°C, the density, the flexural strength, elastic modulus and fracture toughness of the composites are 1.81gcm−3, 39.78MPa, 31.84GPa and 1.46MPam1/2 respectively. Meanwhile, the dielectric constant (ε) and loss tangent (tanδ) of the composites are 2.7 and 0.0052. The microstructural transition from piece bonding to piece stacking can be found as well.

Investigation on field emission properties of graphdiyne–BN composite

In order to explore the electronic structure and field emission properties of the graphdiyne–BN composite, bond lengths, energy levels, local electron density distributions and Mulliken charges are investigated by first principles method. Our research shows that electronic structures of the graphdiyne–BN composite are modified by the effective movement of Fermi level, and the reduction of band gaps with the increasing electric field, local electron density distributions and conduction of Mulliken charges suggest that field emission properties of this composite are improved significantly. At the same time, we find that the binding energy generally increases with the increasing electric field, which contributes to the stability of the graphdiyne–BN composite. In addition, ionizaton potential decreases linearly with the increasing electric field as well as work functions, which further illustrates the enhancement of field emission properties. Through a series of studies, we believe that the graphdiyne–BN composite may become a potential candidate of field emission devices in the future.

Correlations between microstructure and dielectric properties of hexagonal boron nitride

Hexagonal boron nitride is a material with a unique combination of mechanical, chemical, and electrical properties and therefore of considerable technical and commercial interest. Nevertheless, there exists only very limited knowledge concerning the correlation of microstructure and electrical and dielectrical properties of such materials. In this work, the microstructure, dielectric breakdown resistance (dielectric strength), and low permittivity of different BN ceramics and composites were investigated. Besides exhibiting a very high specific electrical resistivity of 10 13 –10 15 Ω cm, the materials had excellent dielectric strengths (up to 53 kV/mm) and low electrical permeability (4.1). The dielectric strength depended strongly on the porosity and to a lesser extent on the content of secondary phases, whereas the permittivity was influenced by the secondary phases. The aging of the materials in humid air did not significantly alter these values. The permittivity was found to be independent of frequency between 0.1 MHz and 10 MHz and temperature up to 300 °C.

From ultrathin nanosheets, triangular plates to nanocrystals with exposed (102) facets, a morphology and phase transformation of sp2 hybrid BN nanomaterials

In this study, an “autoclave route” was applied for the synthesis of 2D sp2 hybrid BN nanomaterials. By simply increasing the reaction temperature, a phase and morphology transformation occurs. The turbostratic BN nanosheets (BNNSs, 2–6 nm, 400–500 °C) transformed into the mixed phases (r-BN and h-BN) of triangular nanoplates (BNTPs) in the temperature range of 550–600 °C, and finally to h-BN nanocrystals (BNNCs, >730 °C). Interestingly, the intergrowths of r-BN and h-BN into the BNTP and BNNC plate was obtained in the range of 600–690 °C, which were determined via the selected area electron diffraction (SAED) pattern. Basically, the sizes (side length and thinness) of BNTPs and BNNCs can be tuned by adjusting the reaction temperature and time. The phase transformation temperature was much lower than that of previous reports. The in situ produced Fe nanoparticles, molten Na and autogenic pressure were considered to have positive effects on the phase transformation. Their band gap was estimated to be 5.6–5.8 eV according to the optical absorption spectrum, and Cathodoluminescence (CL) images show that as-synthesized BN nanomaterials had uniform optical properties in the ultraviolet region. Besides, the Ag–BN composite showed excellent antibacterial activities and therefore has potential applications in biomedical and related fields.

Preparation and characterization of thermally conductive thermoplastic polyurethane/h‐BN nanocomposites

Polyurethane nanocomposites are versatile engineering polymers with unique properties. In this study, nano hexagonal boron nitride containing thermoplastic polyurethane elastomers were prepared via melt blending and hot‐pressing techniques. The nanocomposites were characterized using Fourier transform infrared, differential scanning calorimetry, thermal gravimetric analysis, tensile tests, and thermal conductivity measurements. The surface morphology of the TPU/h‐BN composites was characterized by scanning electron microscopy. The optical properties of the composites were determined by UV transmittance measurements and as the amount of h‐BN increased, optical transparencies decreased dramatically. Nanocomposites displayed higher E‐modulus values and lower elongation at break values than the pure TPU elastomer. Char yields of TPUs increased with increasing h‐BN percentage. Moreover, thermal conductivity of the composite materials improved with the incorporation of h‐BN. POLYM. COMPOS., 35:530–538, 2014. © 2013 Society of Plastics Engineers

High electrical resistivity of pressureless sintered in situ SiC–BN composites

SiC–BN composites with a BN content of 6 wt.% were pressureless sintered to ∼99.4% theoretical density at 2200 °C for 1 h and found to have a high electrical resistivity of 1.24 × 10 10 Ω cm. Starting B 4 C and C were added to react partially with Si 3 N 4 for in situ synthesis of BN and also to serve as sintering aids. Interface diffusion of B and N from BN into SiC was crucial to improving the insulating and dielectric properties through carrier compensation without causing any obvious decrease in the thermal conductivity .

Machinable ZrB2–SiC–BN composites fabricated by reactive spark plasma sintering

A solid state reaction of ZrH2, Si3N4 and B4C has been proposed to synthesize machinable ZrB2–SiC–BN composites. Dense ZrB2–SiC–BN composites with fine grain size and homogeneous microstructure were fabricated via spark plasma sintering at 1900°C in vacuum. The kinetics of the reaction process was studied with XRD results. SEM and TEM results showed that the in-situ formed BN phase was composed of micro-sized intergranular and nano-sized intragranular h-BN particles. The influences of the BN content on the mechanical properties as well as the thermal conductivities of the composites were studied. The composites that contained >20vol% BN exhibited excellent machinability and high strength.

Synthesis of Si3N4–BN composites using borazine as the precursor

Si3N4–BN composites were fabricated via die pressing and PIP route using borazine as the precursor. The composites are composed of h-BN, α-Si3N4 and β-Si3N4 with 3.46wt% α to β phase transition. Desirable mechanical properties of the composites were achieved and special interfacial microstructures can be observed.

Effect of ZrO2 content on microstructure, mechanical properties and thermal shock resistance of (ZrB2+3Y-ZrO2)/BN composites

(ZrB2+3Y-ZrO2)/BN composites were fabricated by hot-pressing with different contents of 3Y-ZrO2 ranging from 0 to 30vol%. The mechanical properties and thermal shock resistance of composites were systematically investigated. The mechanical properties of the composites exhibit a gradual degradation while the thermal shock resistance is improved with the increasing ZrO2 content. The flexural strength and fracture toughness of 30vol% ZrO2/BN composite are 183MPa and 3.80MPam1/2, respectively. The residual strength ratio claims a slight decline to the lowest value of 86.8% after thermal shock, which is improved mainly due to the lower elastic modulus and higher ratio of fracture toughness to flexural strength with increasing ZrO2 content. Furthermore, when temperature difference ranges from 1100 up to 1400°C, the residual flexural strength of the composites increases, which is attributed to the self-healing of cracks and the thermal-barrier effect of the oxide layer.

Microstructure and mechanical properties of ZrB2–SiC–BN composites fabricated by reactive hot pressing and reactive spark plasma sintering

ZrB2–SiC–BN composites were fabricated from a powder mixture of ZrH2, Si3N4 and B4C via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS). The phase composition, density, microstructure and mechanical properties of the final products were studied and compared. The sample fabricated by the R-SPS process showed higher density, flexural strength and fracture toughness and had a more homogeneous microstructure compared to the one fabricated by the RHP process.

Inhibiting Effect of Additives on Formation of ZrC Phase in ZrB2–BN Composites by Reactive Hot Pressing

The ZrB2–BN composite without ZrC impurity was prepared successfully using reactive hot pressing via carbothermal reduction of ZrO2 with B4C, C, and BN powders. The influences of additives and sintering temperature on the phase and microstructure of composites were investigated. The formation reaction of ZrC was inhibited by decreasing the evaporation of intermediate product B2O3 due to forming a SiO2–B2O3 eutectic system with the addition of SiO2 or SiO2–CaF2. In comparison, SiO2–CaF2 exhibited better inhibition capacity of ZrC. ZrB2–BN composite without impure ZrC phase was achieved. In addition, the content of ZrO2 decreased with increasing sintering temperature, and finally disappeared when sintered at 1800°C. Moreover, the relative density of the ZrB2–BN composite reached 96.9%.

Preparation and mechanical properties of SiO2f/BN composites with quasi-sandwich structure for wave transparency

Silica fibers reinforced boron nitride composites with a quasi-sandwich structure for high-temperature wave transparency were fabricated by precursor infiltration pyrolysis technology. The loading capacity of the quasi-sandwich structure is close to that of the solid samples as theoretically calculated, in addition to showing enhanced electrical properties and lower density. The test results for the mechanical properties are basically consistent with the theoretical values. The bending strength of the longitudinal and latitudinal I-shaped samples was 74% and 69% that of the solid sample. The fracture behavior of longitudinal and latitudinal I-shaped samples was also investigated by using compression testing. There are two types of fracture behaviors, which vary with the weaving direction. In the case of longitudinal samples, solid and I-shaped samples share a similar fracture appearance, while the latitudinal ones show crack expansion in the opposite direction between the solid and I-shaped samples. The dielectric constant of the quasi-sandwich is much lower than that in the solid samples, which is very beneficial in terms of the wave-transparent properties of structures such as radomes.

High-temperature properties and associated structure evolution of continuous SiNO fiber-reinforced BN composites for wave transparency

Continuous SiNO fiber-reinforced boron nitride (SiNOf/BN) composites for high-temperature wave transparency have been fabricated by a precursor infiltration pyrolysis (PIP) process using borazine as the BN precursor. The evolution of the properties of the composites at elevated temperatures have been measured in terms of strength, dielectric constant, and loss tangent, and the associated structure evolution has been investigated by XRD, SEM, HRTEM, and FTIR. A flexural strength of 138MPa was maintained up to 1200°C. Deterioration of the mechanical properties at elevated temperature was due to crystallization of the amorphous SiNO fibers into Si2N2O and internal pore formation, coupled with interface reactions. The composites display a low dielectric constant of 3.38 and a low loss tangent of 0.0017, which increases slightly with temperature due in part to the crystallization into Si2N2O and high-temperature polarization. The high-temperature mechanical properties and good dielectric properties of these composites make them useful for wave transparency.

Thermal behavior and dielectric property analysis of boron nitride‐filled bismaleimide‐triazine resin composites

AbstractBismaleimide‐triazine (BT) resin/hexagonal boron nitride (h‐BN) composites are prepared, and the effects of h‐BN content on the thermal and dielectric properties are studied in the view of structure–property relationship. It is found that the introduction of the BN in the BT resin dramatically improve the thermal conductivity of BT resin. The thermal conductivity of the composites is up to 1.11 W/m.K, with an h‐BN concentration of 50 wt %, which is increased by six times compared with the pure BT resin. The BT resin/h‐BN composites also exhibit excellent thermal properties, with the glass transition temperatures above 200°C, and thermal decomposition temperatures over 390°C. Moreover, the composites possess good dielectric properties. Their dielectric constant and loss tangent (tan δ) are less than 4.5 and 0.015, respectively. The results indicate that the BT resin/h‐BN composites are promising as efficient heat‐releasing materials in the high‐density electronic packaging technology. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Microstructure and tribological behaviors of C/C–BN composites fabricated by chemical vapor infiltration

In this paper carbon fiber reinforced carbon–boron nitride binary matrix composites (C/C–BN) were prepared by chemical vapor infiltration (CVI). The infiltration of BN in the CVI process was controlled by the diffusion of BCl3, and BN matrix was distributed homogeneously in the porous carbon fiber reinforced carbon matrix composites (C/C) due to the good infiltration ability of BN. The as-received C/C–BN composites were composed of 92vol% C and 8vol% BN. Both the friction coefficient and wear rate of C/C composites decreased significantly by the incorporation of BN. After heat-treated at 1600°C, the interlayer spacing of CVI BN decreased to 3.36Å, and CVI BN with high crystalline degree displayed the excellent lubricating effect, leading to the decrease of friction coefficient and wear rate. The improvement of the tribological properties also was partially attributed to the improved oxidation resistance and the formation of friction film by the incorporation of BN matrix.

Synthesis and Characterization of Chitosan/Boron Nitride Composites

Chitosan/Boron nitride (BN) composites were prepared by solution method using CuSO4/glycine chelate complex as the catalysis with variable percentage of boron nitride loading. The dispersion of BN with the chitosan polymer was achieved with sonication at power of 120 W and frequency of 80 kHz. The chemical interactions of chitosan and BN were studied by Fourier transform infrared spectroscopy. The structure of chitosan and BN composites were investigated by XRD and TEM. It was observed that the BN were dispersed with chitosan matrix through intercalation. The quantitative identification of composites was investigated by Energy Dispersive X‐ray Spectroscopy and Selected Area Electron Diffraction. Thermal stabilities of chitosan/BN composites were studied by thermogravimetric analysis. It was found that, the thermal stability of the chitosan/BN composites was increased compare with virgin chitosan. The oxygen barrier properties of chitosan/BN composites were measured using gas permeameter. A substantial reduction in oxygen permeability was observed increasing boron nitride concentrations by which the synthesized composite materials may applicable in packaging industry.

Thermal Behaviours and Single Cell Performance of PBI‐OO/PFSA Blend Membranes Composited with Lewis Acid Nanoparticles for Intermediate Temperature DMFC Application

AbstractBuilding on the improved proton conductivity at intermediate temperatures under anhydrous conditions using a ‘proton donor–proton acceptor' concept demonstrated for the first time in the composite membrane made of inorganic lewis acid (boron‐based electron‐deficient nanoparticles) and a poly‐[(1‐(4,4′‐diphenylether)‐5‐oxybenzimidazole)‐benzimidazole] (PBI‐OO)/perfluorosulfonic acid (PFSA) blend, we report here the thermal behaviours and direct methanol fuel cell (DMFC) single cell performance of these membranes at intermediate temperatures (90–120 °C). It is found that the PFSA/PBI‐OO blends and PFSA/PBI‐OO/nano‐BN composites show decreased thermal stability compared to pristine PFSA. We attribute this to a proton transfer reaction between the sulfonic acid and imidazole moieties of the constituent polymers inducing a decreased stability of the resulting sulfonate group. When operated with dry oxygen and methanol/water vapour, the single cell performance of PFSA/PBI‐OO blends is slightly improved compared to that of pristine PFSA. The PFSA/PBI‐OO/nano‐BN composite membranes exhibit much better single cell performance at intermediate temperatures, which could be mainly attributed to its higher anhydrous proton conductivity.

Production and properties of AlN–BN composite

The nitridation of an AlB2 precursor in a controlled nitrogen flow is examined with the aim of obtaining AlN–BN composite powders. It is shown that superfine powders with uniformly distributed AlN and BN phases and AlB12 admixture form over the temperature range 1000–1400°C. The powders do not require further grinding. The morphology and particle size of the powders and the microstructure and properties of hot-pressed samples are studied. The developed material exhibits better mechanical properties (as compared with known ABN composites): in particular, Rbm = 235 MPa and HV = 14.2 ± 1.3 GPa. The high bending strength and hardness are due to the finegrained structure of the samples and the strengthening effect of superfine (≤1 μm) inclusions of aluminum dodecaboride.

Effects of cubic BN addition and phase transformation on hardness of Al 2O 3–cubic BN composites

The Vickers hardness of dense Al 2O 3–cubic BN (cBN) composites prepared by spark plasma sintering under a moderate pressure of 100 MPa at 1200–1600 °C was investigated at indentation loads of 0.098–19.6 N. The BN grains in the Al 2O 3–BN composite prepared at 1300 °C showed no transformation from the cBN to hBN phase, and the hardness was 59 GPa at 0.098 N. The hardness of the Al 2O 3 matrix in the Al 2O 3–BN composites containing 10–30 vol% cBN prepared at 1300–1400 °C was around 25 GPa at 0.098 N, which was higher than monolithic Al 2O 3 bodies prepared at the same temperatures. The hardness of the Al 2O 3 matrix in the Al 2O 3–BN composites decreased with increasing sintering temperature. The increase in the hardness of the Al 2O 3 matrix may be due to the decrease in the size of Al 2O 3 grains in the Al 2O 3–BN composites owing to the addition of cBN particles and the decrease in sintering temperature. The Meyer exponents of the monolithic Al 2O 3 bodies and Al 2O 3–BN composites were 1.90–1.94 independent of cBN content.

An investigation on electro discharge micro-drilling of SiC-20% BN composite

SiC-20% BN composite is a newly developed material having potential applications in several engineering industries. Machining of such type of advanced ceramic composites by conventional methods is not only difficult to perform but it is also very costly. Electro discharge machining is seen as a potential process for such materials because it does not exert any mechanical force on tool and work piece and it is also independent of hardness of the work piece. In the present research the characteristic features of electro discharge micro-drilling of SiC-20% BN composite are studied through experimental investigation. The measured responses are material removal rate (MRR), tool wear rate (TWR) and relative electrode wear (REW). The optimal parameter settings for the responses are determined with the help of a statistical analysis. SEM micrographs of the holes were taken to illustrate the material removal mechanism of the ceramic composite. In the present research study it has been proved that EDM is a potentially successful technique for micro-hole drilling in this new generation composite.